Mark-sensing photosensor

ABSTRACT

A periodically oscillating electro-optical scanning apparatus for indicating the position of registration lines or edges relative to itself by providing an electrical signal output indicative thereof. The optical scanner is positioned relative to the material containing the indicia to eliminate output signal deficiencies.

United States Patent Willits Feb. 15, 1972 [54] MARK-SENSING PHOTOSENSOR[56] References Cited [72] Inventor: Samuel P. Willits, Barrington, lll.UNITED STATES PATENTS Assignee= Spartanics Harrington Village, 3,335,2818/1967 Willits ..250/202 [22] Filed: Jan. 29, 1970 b Primary ExaminerRoert Sega] [21] Appl' 6897 Att0rneyJacque L. Meister Related US.Application Data [57] ABSTRACT [62] Division of Ser. No. 650,835, July3, 19 A periodically oscillating electro-optical scanning apparatus forindicating the position of registration lines or edges rela- [52] us CL"Mao/202, 250/235, 250/219 R tive to itself by providing an electricalsignal output indicative 51 Int. Cl ..G05k 1/01, GOlj 1/36 t T opticalscanner is positioned relative to the [58] Field of Search ..250/202,203, 219 R, 2l9 Q, material containing the indicia to eliminate outputsignal defi- 250/219 QA, 234, 235 ciencies.

5 Claims, 4 Drawing Figures SIGNAL AMPLIFIER FILTER 5a g PHASE fioSHIFTER J 3 v .96

P5122125 8w 1 M, CRCU'T DETECTOR LEVEL w 1 29/ 1 72 REF E LEVELS re sCONTROL .3ga RELAY AUXILARY CONTROL SIGNAL PATENIEDFEB 15 I972 sum 1 BF2 POWER AMPLIFIER 54 I 2 52a 2 2F SIGNAL AMPLIFIER FILTER PHASE SHIFTERSIGNAL PRESENCE DETECTOR EVEL CONTROL RELAY AUXILARY CONTROL SIGNAL ATTYSamuel P.WiIliIs IN NTOR.

MARK-SENSING PHOTOSENSOR OTHER APPLICATIONS This application is adivision of the application of SAMUEL P. WILLITS et al., Ser. No.650,835, filed July 3, I967, titled MARK SCANNING PHOTO SENSOR AND SERVOSYSTEM.

BACKGROUND OF THE INVENTION The field of the invention is generallyrelated to improvements in apparatus for automatically positioningregistration marks or indicia and, more particularly, to aneIectro-optical sensing apparatus having outputs either useful forpositioning the apparatus or elements associated therewith relative tothe registration indicia or for providing indications of the excursionthereof.

In an earlier application of Samuel P. Willits, now issued as US. Pat.No. 3,335,28l, an apparatus is described which is responsive toregistration indicia having linelike width characteristics relative tothe effective width of the active electro-optical elements of thesensing head. In that apparatus the electrical signal output of acyclically scanning sensing head was amplified and filtered to provide acontrol signal for a servo positioning device which mechanicallyrepositioned the linelike registration indicia into a preselectedalignment with the sensing head and, when this alignment was perfected,generated an auxiliary control signal indicative of satisfactorycompletion of the repositioning. The auxiliary control signal typicallywould be used for initiating a further operation of an associatedmachine such as tripping of a shear blade, actuation of a ram, etc.

A particular feature of the Willits patent is a special angularrelationship of the material and indicia to the scan direction and lightsource This special relationship has several advantages includingelimination or reduction of errors caused by parallax and shadow effectsor signal polarity reversals during a scan. In some embodiments, novelimage offsetting means are used to advantage.

Various other devices are known for indicating the position of alinelike registration indicia and for servo positioning in responsethereto. In some of these there is no movement of the scanning elementand detection requires the movement of the registration indicia past theelements to evoke a signal. A device typical of this'type is disclosedby J. C. Frommer in US. Pat. No. 2,840,371, issued June 24, I958.Frommers device provides repositioning of plural indicia upon a movingstrip relative to his sensor and thus maintains registration duringcontinuous feed color printing operations.

In other known devices there is movement of the scanning elements makingmovement of the indicia an unnessary prerequisite to detection. Devicestypical of this type are described by M. A. McLennan in US. Pat. No.2,489,305, issued Nov. 29, I949, and W. P. Frantz in US. Pat. No.2,892,948, issued June 30, I959. The Frantz device however, unlike theothers, requires the presence of two registration indicia to enable itto provide a suitable servo output signal.

Included in the prior art is the disclosure of H. R. Summerhays, Jr. inUS. Pat. No. 2,674,917, issued Apr. l3, I954. Summerhays discloses anoncontacting electro-optical width gauge incorporating fixed prisms andmirrors, wieldable independently of the prisms, to effect a scan of theedge of strip material moving beneath the prisms and mirrors. Noapparatus is disclosed for any automatic repositioning, the deviceproviding only meter indications of strip width variations.

From the foregoing brief discussion of prior art devices taken inconjunction with their complete disclosures, it can be seen that theprior art makes no provision for eliminating parallax errors due topositioning of the electro-optical sensor relative to the registrationindicia nor is there provision for eliminating shadow effects such asoccur when the indicia is either depressed below or elevated above thesurface bearing it. The inability of the prior art to overcome parallaxerrors or shadow effects are overcome by the present invention.

SUMMARY OF INVENTION A principal object of the invention is theelimination of parallax errors, shadow effects and polarity reversals inthe output signals of a periodically oscillating electro-opticalscanning apparatus. This object is realized by maintaining a scanningradiation source and oscillating scanning sensor in a special angularrelationship to each other and a registration indicia and the materialupon which the indicia is located.

Still another object of the invention is to provide new and improvedelectro-optical sensing apparatus having wieldable optical means forphysically offsetting the image of the scanned registration indicia afixed distance from a plane containing the optical axes of the source ofscanning radiation and the scanning sensor. This object is achieved byplacing two reflecting surfaces, which desirably are supplied by arhomboid prism, in the optical paths between the indicia and combinationof sensor and light source.

The foregoing and other objects of the invention are achieved by a novelelectro-optical sensing head which produces electrical signals havingcharacteristics related to the relative position of the registrationindicia or mark. The scanning radiation source and oscillating scanningsensor are maintained in very special angular relationships with respectto each other and the registration indicia and material bearing it whilethe scan is taking place.

The nature of the invention and its several features and objects willappear more fully from the following description made in connection withthe accompanying drawings.

DESCRIPTION OF DRAWINGS FIG. 1 is a combination mechanicalschematic-electrical block diagram of the principal elements of theinvention;

FIG. 2 is a waveform diagram illustrating output waveforms from theinventive sensing head for the various positions of the registrationindicia relative thereto illustrated in FIG. 3 and of the waveformsappearing at the indicated point in the circuitry of FIG. 1;

FIG. 33 through 3F illustrate various positions of a registration marksensed by the invention relative to the active scanning area of theinvention; and

FIG. 4 is a partial mechanical-optical schematic of an embodiment of thesensing head of the invention showing the method of achieving an offsetoptical path.

DESCRIPTION OF INVENTION The present invention utilizes elements of theaforementioned Willits application. Accordingly, FIG. I illustrates insimplified schematic form the principal elements of the Willits sensingapparatus. There, material 20, bearing a suitable regis tration mark 22is positioned to be within the field of view of a photodetector 24mounted on an oscillating arm 26. Arm 26 is oscillated about pivot axis28 through an angle a between the two fixed positions indicated inphantom outline at 26' and 26" by a conventional moving coil type ofelectromechanical drive schematically indicated by arm drive coil 30.Coil 30 is excited by an amplified AC reference signal applied atterminal 58. The combination of oscillation and the physical size of theactive area of detector 24, results in a scanned area indicated at 32.

A light source 34 is focused by condensing lens 36 on scanned area 32 toprovide illumination for detector 24. Suitable aperture stops (notillustrated) may be used in some instances to restrict the light fromsource 34 to scan area 32 or nearly so, although such steps are notessential to the invention. The only requirement in this regard is theprevention of direct irradiation of detector 24 by source 34. Lightsource 34 is excited by a DC source 38 to insure no AC signal componentswill be generated by detector 24 by any rapid variations in intensity ofthe source. Use of such a DC excited light source has provenadvantageous as will become apparent later in this description.

An image of the active scanning area 32 is formed by objective lens 40in or substantially in the plane of movement of oscillating arm 26 andspecifically in the plane of an aperture mask 42 positioned on that armin front of detector 24. To provide an enhanced signal amplitude and anaveraging effect to overcome any problem created by any variations inline width or smoothness, the shape of the aperture in mask 42 has beenmade long and narrow with the major axis of the aperture positionedparallel to the axis of scan arm 26. Detector 24 is positioned relativeto the aperture in mask 42 so that only light passing through theaperture from lens 40 can fall on the detector's sensitive surface. Inthis manner the aperture in mask 42 defined the active area of detector24. In the majority of embodiments constructed, the detector employedhas been a silicon photovoltaic cell. This particular type of cell waschosen for its small size, logarithmic saturation characteristics andlow impedance which matches the transistorized signal processingcircuitry employed. Obviously, however, other types of cells may beemployed depending on operating parameters.

In operation, oscillating arm 26 oscillates through a small angle aabout axis 28 in a plane perpendicular to the optical axis 44 of lens40. While the absolute scan amplitude 47 plus the width of the aperturein mask 42 is necessarily greater than the imaged width of registrationmark 22, the showing of FIG. 1 has been exaggerated for illustrationpurposes. More nearly to scale is the illustration of FIG. 3 discussedbelow. The elements of the scanning head are so arranged that thisoscillation of arm 26 periodically translates aperture mask 42 anddetector 24 across the image of active sensing area 32 in a directionsubstantially perpendicular to the long axis of the image ofregistration mark 22. Thus, at any instant, the light falling ondetector 24 through the aperture in mask 42 is a measure of thebrightness of the particular part of the image of the active sensingarea visible through the aperture in the mask. The resulting electricaloutput of detector 24 is thus proportional to the brightness of aparticular part of the image of active sensing area 32 falling at anyinstant on the aperture in the mask 42.

The signal outputs of detector 24 for various positions of registrationmark 22 relative thereto are discussed in conjunction with FIGS. 2 and3. However, before continuing with a description of these signals and ofthe signal processing circuitry, a description and understanding orother characteristics of the scanning head of the invention will aid inunderstanding the detector signal outputs and processing circuitry.

In certain applications, the material 20 bearing the registration marks22 has a highly polished surface. In such an instance, the specularreflections from the polished surface contain a very high percentage ofthe total light reflected from that surface. Additionally, depending onthe angle of the optical axis 46 of lens 36 relative to the polishedsurface and the angle of optical axis 44 relative thereto, the image ofthe registration mark will appear either light or dark regardless of itsabsolute contrast relative thereto. Obviously this can present problemsof signal polarity as the material 20 is positioned. To provideconsistent polarity of the detector signal in the face of such acondition and to enhance signal levels and reduce spurious signals, ithas been found desirable to have the optical system, including thesurface of material 20 in a particular optical alignment that alsomaximizes specular reflections. l have found that this particularoptical alignment can be achieved by arranging optical axes 44 and 46 insuch a manner as to cause them to intersect each other at or very nearthe surface of material 20, by having axes 44 and 46 and a line 48positioned between the axes in the same plane, and by further arrangingthe axes 44 and 46 relative to line 48 so that the angles I formedbetween each axis and that line are equal.

Another type of material variation that can affect the accuracy ofscanner type sensing apparatus of the type employed in my invention,occurs when the distance from the sensing head to the registration markvaries. This distance variation most frequently is caused by materialthickness changes, but also may be caused by raised or depressedregistration indicia 22 causing shadow effects. Either materialthickness variations or shadow effects would ordinarily cause errors inmeasurement of indicia location which errors can result in positionalerrors of the work piece 20 relative to the desired reference position.It is an advantageous feature of my invention that these positionalerrors caused by parallax or shadow effects are eliminated by aparticular geometrical arrangement of the entire illuminating, imagingand scanning system. The particular geometrical arrangement usedoperatively places these elements in a plane that contains the axis ofthe registration indicia and that is mutually perpendicular to both thescanning direction and the surface of the material 20 being registered.Such an arrangement is shown in FIG. I.

In many applications of my automatic sensing apparatus, mechanicalinterference with machine elements physically prevents achieving theabove set forth requirement for operating the active elements of thesensing head in a single plane which includes the registration mark. Toovercome this difficulty and retain the advantages of operating theelements of the sensing head in a single plane, we provide an opticaloffset achieved by the insertion of a rhomboid prism into the opticalsystem near or adjacent to the surface bearing the registration mark.This arrangement is illustrated in FIG. 4 where rhomboid prism 50 isshown positioned adjacent active scanning area 32 and material 20 toprovide the desired offset in optical axes 44 and 46 relative to fixedshear blade 52 and movable shear blade 54. As will be apparent to thoseversed in the optical arts, reflectors and other optical elements can beused to obtain optical offsets in place of the illustrated rhomboidprisms and such elements are employed when the optical path is modifiedto reduce its height. However, a special feature of a rhomboid prism isthat optical systems employing such a prism are relatively insensitiveto small positional changes of the prism relative to the sensing head.

In certain applications of my automatic sensing apparatus, somemechanical part or machine element, such as a shear blade, must be movedthrough the space occupied by the opti cal system of the invention. Itis a special feature of the invention that such movements of machineelements can be effected without resultant damage to the optical system.This advantageous result is obtained by utilizing a wieldable rhomboidprism in the inventive optical system. Because of the properties of therhomboid prism, the prism can be reciprocally displaced in thedirections of double ended arrow 56 during the shear cycle and any smallpositional changes in prism location to the optical axes 44 and 46 thatoccur when the prism is returned to normal operating position afterdisplacement do not result in any error in location of the referenceposition on subsequent cycles. As will become apparent from descriptionwhich follows, prism 50, when necessary, may be moved to clear a movingmachine element when an output signal which actuates the shear isdeveloped in the circuitry of the invention. The apparatus to move prism50 has not been illustrated since it is conventional in nature and formsno part of my invention. However, one mechanism successfully employedfor this purpose consisted solely of a solenoid-actuated bellcrank towhich prism 50 was cemented. Other mechanisms may be employed as welldepending largely on the type of machine tool with which my inventiveapparatus is associated.

FIGS. 28 through 2F illustrate the output waveforms of detector 24 forthe various positions of the registration mark as illustrated at thecorrespondingly lettered subfigure of FIG. 3. In FIG. 2, time increasesfrom left to right. FIG. 2A illustrates the time-varying position ofoscillating arm 26. Because of the very low inertia of arm 26, wavetrain 2A may also be considered to be identical or nearly identical withthe AC reference voltage supplied to the system circuitry at terminal58.

When registration mark 22 is displaced from the center 60 of thescanning area 32 to a position to the left and near one edge thereof asshown in FIG. 3B, the output signal of detector 24 is shown in FIG. 2B.The flattop or straight line portion of the wave train occurs when thedetector scans the image of the uniform background 62 of materialadjacent mark 22. The negative-going loop occurs once each cycle as thedetector cyclically moves into and out of the image of the registrationmark. Note that in this and the following examples, the referenceposition with respect to which registration mark 22 is positioned, hasbeen assumed to be the center 60 of the active scanning area 32. Otherreference positions, offset as desired from the position of thisexample, could be employed. However, normally the reference position islocated at the center of the active scan area.

If the registration mark is then positioned nearer to the center 60 ofactive scanning area 32 as shown in FIG. 3C, detector output is as shownin FIG. 2C. There again the straight line portion of the wave trainoccurs when the detector does not see" the registration mark and onlysees the uniform background area 62 of material 20. The doublenegative-going loop is generated as the detector cyclically scans backand forth through the image of the registration mark 22. Note that whencompared with FIG. 2B the duration of the scan through the image of themark 22 has increased relative to the duration of the scan of theuniform background area 62 and that the two scan durations have almostbecome equal.

When the registration mark 22 is positioned at the center 60 of theactive scanning area as illustrated in FIG. 3D, the output wave train ofdetector 24 is as illustrated in FIG. 2D. Two complete cycles eachconsisting of a negative-going loop and a straight line portion appearin the output wave train for each scan cycle with identical waveformsgenerated on each side 'of the scan cycle. The output wave train is, forthis position of the mark, composed only of even harmonics of thefrequency of displacement of oscillating arm 26 with no fundamentalcomponent. Thus, the output wave train comprises a signal whosecomponent frequencies, other than the constant term, are all evenharmonics of the frequency of FIG. 2A.

If the registration mark 22 is moved to the right of center 60 ofscanning area 32 as shown in FIG. 3E, the output wave train of detector24 is as illustrated in FIG. 2E. A move of the work further to the rightas shown in FIG. 3F results in output wave train 2F. By inspection ofFIG. 28 through 2F, it can be seen that as the registration mark 22 isdisplaced one way or the other from the center 60 of scanning area 32,the fundamental component in the output wave train appears. Further, asthe displacement of the mark from center increases, the fundamentalcomponent increases in amplitude with its phase determined by thedirection of displacement from center 60. Displacement of the mark oneway produces a fundamental component in phase with the wave train ofFIG. 2A and displacement in the other direction produces a fundamentalcomponent 180 .out of phase with that wave train. Thus, thecharacteristics of the fundamental contained in the output wave trainare exactly those required to provide a servocontrol signal indicativeof the position of registration mark 22. The means by which I convertthese characteristics of the detector signal are illustratedschematically in FIG. I.

The output signal from detector 24 is amplified in signal amplifier 64.Signal amplifier 64 is a cascaded grounded emitter transistor amplifierwhose final stages are operated saturated to maintain a comparativelyuniform servo loop gain over large ranges of signal level from detector24. The output signals of amplifier 64 corresponding to its inputsignals 28, 2C, 2D, 2E and 2F, are also illustrated in FIG. 2 as signalsB, C, D, E and F respectively. The manner of achieving thesecharacteristics of signal amplifier 64 and the reason therefore areexplained in detail in the aforementioned Willits patent. The amplifiedoutput of signal amplifier 64 is processed by low-Q tuned filter 66which passes the fundamental frequency contained therein and reduces theamplitude of any higher harmonics present.

The filtered signal is further amplified in push-pull cascaded emitterfollower transistor power amplifier 68 before being applied to thecontrol winding 72 of two-phase servomotor 70.

Phase shift network 76 provides a signal on winding 74 phased 90 fromthat on winding 72. The servo loop back to indicia 22 is closed byproviding a mechanical link between the output shaft of servomotor 70and material 20. This link is schemati- 5 cally shown as 78 in FIG. 1.

The Willits apparatus of this application also includesposition-analyzing circuits capable of generating an auxiliary controlsignal whenever positioning of a registration mark has beensatisfactorily accomplished. These circuits are described in detail inthe Willits patent and comprise a logic circuit 80 and a signal presencedetector 290. These two circuits together analyze the output signal ofthe power amplifier 68 in terms of voltage references supplied atterminals 87 and 96 and initiate the auxiliary control signal formed bycontrol relay 100 after the passage of time occasioned by time delay 98.Time delay 98 insures that the auxiliary control signal will not beinitiated prematurely as it otherwise might under condition ofservomotor overor undershoot.

While certain specific examples of the invention have been disclosed, itwill be understood that further variations and modifications could bemade in the system that are within the spirit and scope of the inventionas described herein.

I claim:

1. An electro-optical sensing head for providing an output signalindicative of the location of a single registration mark on a surfacepraticularly positioned relative thereto comprismg:

scanning means including a radiation detector cyclically oscillatingover a substantially linear path in a direction predetermined relativeto said registration mark,

a source of radiation,

a condensing lens focusing said radiation on said registration mark,

an objective lens for imaging said registration mark on said radiationdetector, and

the optical axes of said condensing lens and said objective lens eachhaving an identical angular relationship to a line drawn perpendicularto the surface bearing said registration mark and each axis intersectingsaid line at said surface, said optical axes being further positioned tolie in a common plane which is perpendicular to both the scanningdirection and said surface.

2. An electro-optical sensing head for providing an output 45signal-indicative of the location of a single registration mark on asurface particularly positioned relative thereto comprisscanning meansincluding a radiation detector cyclically oscillating over asubstantially linear path in a direction predetermined relative to saidregistration mark,

a source of radiation,

a condensing lens focusing said radiation on said registration mark,

an objective lens for imaging said registration mark on said radiationdetector, and

the optical axes of said condensing lens and said objective lens eachintersecting at said surface, said axes being further positioned to liein a common plane which is perpendicular to both the scanning directionand said surface.

3. An electro-optical sensing head in accord with claim 2 furthercomprising optical means wieldable independently of said objective lensand said condensing lens for physically offsetting the image of saidregistration mark a fixed distance relative to said source of radiationand said scanning means while maintaining the planes containing saidaxes parallel on either side of said offset.

4. An electro-optical sensing head in accord with claim 2 wherein saidpredetermined direction of cyclical oscillation is about an axissubstantially perpendicular to the optical axis of said objective lensand lying in said common plane.

5. An electro-optical sensing head for providing an output signalindicative of the location of a single registration mark on a surfaceparticularly positioned relative thereto comprismg:

is perpendicular to both the scanning direction and said surface,and

optical means wieldable independently of said objective lens and saidcondensing lens for physically offsetting the image of said registrationmark a fixed distance relative to said source of radiation and saidscanning means while maintaining the planes containing said axesparallel on either side of said offset, said wieldable optical meanscomprising a rhomboid prism.

1. An electro-optical sensing head for providing an output signalindicative of the location of a single registration mark on a surfacepraticularly positioned relative thereto comprising: scanning meansincluding a radiation detector cyclically oscillating over asubstantially linear path in a direction predetermined relative to saidregistration mark, a source of radiation, a condensing lens focusingsaid radiation on said registration mark, an objective lens for imagingsaid registration mark on said radiation detector, and the optical axesof said condensing lens and said objective lens each having an identicalangular relationship to a line drawn perpendicular to the surfacebearing said registration maRk and each axis intersecting said line atsaid surface, said optical axes being further positioned to lie in acommon plane which is perpendicular to both the scanning direction andsaid surface.
 2. An electro-optical sensing head for providing an outputsignal indicative of the location of a single registration mark on asurface particularly positioned relative thereto comprising: scanningmeans including a radiation detector cyclically oscillating over asubstantially linear path in a direction predetermined relative to saidregistration mark, a source of radiation, a condensing lens focusingsaid radiation on said registration mark, an objective lens for imagingsaid registration mark on said radiation detector, and the optical axesof said condensing lens and said objective lens each intersecting atsaid surface, said axes being further positioned to lie in a commonplane which is perpendicular to both the scanning direction and saidsurface.
 3. An electro-optical sensing head in accord with claim 2further comprising optical means wieldable independently of saidobjective lens and said condensing lens for physically offsetting theimage of said registration mark a fixed distance relative to said sourceof radiation and said scanning means while maintaining the planescontaining said axes parallel on either side of said offset.
 4. Anelectro-optical sensing head in accord with claim 2 wherein saidpredetermined direction of cyclical oscillation is about an axissubstantially perpendicular to the optical axis of said objective lensand lying in said common plane.
 5. An electro-optical sensing head forproviding an output signal indicative of the location of a singleregistration mark on a surface particularly positioned relative theretocomprising: scanning means including a radiation detector cyclicallyoscillating over a substantially linear path in a directionpredetermined relative to said registration mark, a source of radiation,a condensing lens focusing said radiation on said registration mark, anobjective lens for imaging said registration mark on said radiationdetector, the optical axes of said condensing lens and said objectivelens each being positioned to intersect at said surface and to lie in acommon plane which is perpendicular to both the scanning direction andsaid surface, and optical means wieldable independently of saidobjective lens and said condensing lens for physically offsetting theimage of said registration mark a fixed distance relative to said sourceof radiation and said scanning means while maintaining the planescontaining said axes parallel on either side of said offset, saidwieldable optical means comprising a rhomboid prism.